Better JBacon.once/interval. Bugfixes

JBacon.once behaves correctly now: it sends an Event.Initial with the
given value, then an Event.End.

JBacon.interval now uses the Timer class to schedule events. While this
does use a background thread, it's cleaner and designed to have lots of
scheduled tasks without having to manage them on my own.

There is now some debug output in various places to help ensure stuff
is being properly ended/removed/etc.

Main.java has some more example code for now.